Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Recommended array size
7.18
Panel count
18
Roof area needed
324
Estimated annual production
11,000
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How to use this
- 1Enter annual electricity usage (kWh).
- 2Enter peak sun hours per day (hrs).
- 3Enter system loss factor (%).
- 4Enter panel wattage (W).
- 5Enter target usage offset (%).
- 6Read your recommended array size on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
Sizing a solar system starts with your actual annual usage, not a rule of thumb. This calculator divides your annual kWh by your location's average sun hours and a system-loss factor to get the DC array size, then converts that into a panel count using the wattage you're quoting. It accounts for inverter and wiring losses (typically 14-18%) and lets you target less than 100% offset if your utility's net-metering rules make overproduction pointless. Real installers use this same math — PVWatts-style production estimate — before ever climbing on a roof. The output includes roof area needed assuming standard 18 sq ft panels, so you can sanity-check against your actual roof before requesting quotes.
Worked example
Using the values the calculator loads with:
Inputs
- Annual electricity usage: 11000 kWh
- Peak sun hours per day: 5 hrs
- System loss factor: 16 %
- Panel wattage: 400 W
- Target usage offset: 100 %
Results
- Recommended array size: 7.18
- Panel count: 18
- Roof area needed: 324
- Estimated annual production: 11,000
What each field means
Inputs
- Annual electricity usage (kWh)
- The annual electricity usage used in the calculation, measured in kWh. Starts at 11000 kWh so you have a working example on load.
- Peak sun hours per day (hrs)
- The peak sun hours per day used in the calculation, measured in hrs. Starts at 5 hrs so you have a working example on load. Accepted range: 2–7 hrs.
- System loss factor (%)
- The system loss factor used in the calculation, measured in %. Starts at 16 % so you have a working example on load. Accepted range: 5–30 %.
- Panel wattage (W)
- The panel wattage used in the calculation, measured in W. Starts at 400 W so you have a working example on load. Accepted range: 200–600 W.
- Target usage offset (%)
- The target usage offset used in the calculation, measured in %. Starts at 100 % so you have a working example on load. Accepted range: 50–150 %.
Results
- Recommended array size
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Panel count
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Roof area needed
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Estimated annual production
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Why not just size for 100% offset?
Most utilities only credit excess production at wholesale or avoided-cost rates, sometimes near zero. If your net-metering agreement doesn't bank kWh at retail value, oversizing past 100-110% offset wastes capital on panels that pay back slowly. Check your utility's interconnection rules before targeting more than full offset.
What sun-hour number should I use?
Look up your city's PVWatts average or NREL solar resource map. Arizona and the Southwest run 6-6.5 hours; the Pacific Northwest and Northeast run 3.5-4.5. Using a national average of 5 will over- or under-size your system by 20-30% depending on where you live.
Does panel orientation change this number?
Yes — this calculator assumes a reasonably good south-facing roof. East/west splits or steep tilt mismatches cut production 10-20%, which you should fold into your sun-hours input or check with the roof-orientation derate tool.
How much does system loss really vary?
16% is a solid default covering inverter conversion (2-3%), wiring (2%), soiling (2%), shading (varies), and temperature derate (3-8% in hot climates). Microinverters and shade-heavy roofs push this toward 20-25%.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Solar Payback Calculator (with ITC)
Years to break even after the 30% federal solar tax credit.
Roof Orientation & Tilt Solar Derate Calculator
How much azimuth and tilt cost you versus a perfect south-facing roof.
Solar Panel Degradation Over 25 Years Calculator
Project your system's output decline over its warranty life.
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Solar Array Sizing Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/solar-array-sizing
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/solar-array-sizing" target="_blank" rel="noopener">Solar Array Sizing Calculator — RevenueLab</a> (2026).</p>
Source: [Solar Array Sizing Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/solar-array-sizing) (2026).